This research project aims to gain a better understanding of the biology of teratocarcinomas of the testis and ovary. We have developed and are developing new recombinant inbred (RI) strains with high and low susceptibility to teratocarcinogenesis. In both males and females, teratomas are derived from germ cells that begin to develop parthenogenetically. We have developed several new RI lines, each derived from two progenitor strains. Some of these lines are even more susceptible than either parent strain to teratocarcinogenesis. We dissociated the cells of male genital ridges, separated the somatic cells from the germ cells, and reconstituted testes with various combinations of age and genotype. The genotype and age of the germ cell is crucial to the formation of teratomas.